Electronic control system



E. L. DEI-:TER 2,503,248

ELECTRONIC CONTROL SYSTEM 5 Shets-Sheet l April 11, 195o Filed Deo. 18, 1945 v wcm/Km EL.eeZr

April 11, 1950 L.. Dl-:ETER 2,503,248

ELECTRONIC CONTROL SYSTEM Filed Dec. 18, 1945 3 Sheets-Sheet 2 EL.De'eer SHV/mm3 E. L. DEETER ELECTRONIC CONTROL SYSTEM April ll, 1950 3 Sheets-Sheet 5 Filed Dec. 18, 1945 Flag 6.

Syvum Elhreer 40 v m '20 |60 m 240 2&0 320 Patented Apr. 1l, 1950 UNITED STATES PATENT OFFICE (Granted under the act of March 3, 1883, as amended April 30, 1928; 370 O. G. 757) 5 Claims.

The present invention relates generally to control devices. More particularly, the present invention relates to an electronic control device adapted to respond to minute changes in a condition to be controlled.

It is well known that if the control grid of an ordinary radio receiving tube is left disconnected, that is floatingf and proper operating potentials -are applied to the plate and cathode thereof, the

tached to the grid cap and the grid to cathode impedance of the tube used. As the negative charge on the control grid increases, the plate current flowing in the tube decreases whereby the value of plate current is controlled by variations in the strength of the ambient electro-static eld.

The floating grid principle above described is utilized in the device of the present invention to vprovide stepless control over changeable conditions. When an alternating potential is applied to the control grid of a tube through a small condenser, normal operating potentials being applied -to the remaining elements thereof, electrons from the control grid collect on that condenser plate which is located nearest the grid during the positive half cycle. During the next half cycle when the outer condenser plate is negative, the electrons accumulated during the previous half cycle are trapped in the circuit between the inner condenser plate and grid. This accumulation of electrons continues on the following cycles until the ycharge on the grid becomes sufl'iciently negative to effectively block the flow of plate current within the tube. The extent of the blocking of plate current accomplished in this manner is dependent upon the capacity of the condenser, the

value of A.C. potential applied to the grid and the leakage resistance from grid to cathode or ground over the grid supports and surface of the tube. Thus, a change in the value of applied A.-C. potential or in the capacitance of the condenser changes the grid bias of the tube and thereby controls the value of current flow through the tube.

In accordance with the preferred embodiment of the present invention, a third plate is introduced into the condenser and is supported on the indicating arm of a, meter whereby the third plate is adapted to pass between the inner and outer plates of the condenser to reduce the capacity thereof as the meter is actuated. The movable plate is tied to the cathode of the tube through ground whereby the blocking bias of the tube associated with the condenser is reduced as the movable plate passes between the inner and outer condenser plates thereby controlling the value of plate current in the tube. This change in plate current is amplified and utilized to operate a suitable gas tube which, when operated, is adapted to energize an electrical relay. The aforesaid meter may be connected to any suitable device responsive to a changeable condition which it is desired to control, such a device, for example, being a thermocouple when it is desired to utilize the present invention for temperature control. As the electrical relay is energized, a, circuit is completed whereby an indication of a change in thetcondition to be controlled may be made manifes In accordance with another embodiment of the present invention, a system for driving a recorder in response to relatively Weak signals is contemplated which incorporates the aforedescribed grid blocking principle. A pair of vacuum tubes are arranged in a bridge circuit with the blocking potentials applied to the grids thereof in a pushpull connection. The change in plate current produced in each tube by changes in the blocking potentials is utilized to control the operation of the recorder driving motor.

It is a broad object of the present invention to provide new and improved means for controlling the electron flow in an electron discharge device.

Another object is to provide new and improved means for producing and controlling an electron accumulation on the control element of an electron discharge device.

Another object of the present invention is to provide a new and improved stepless control device Which is continuously responsive to changes in a condition to be controlled.

Another object is the provision of a new and improved electronic control devioe adapted to operate a relatively heavy mechanism in response to small changes in a condition to be controlled.

A further object is the provision of a control device of the electronic relay type in which an instantaneous operating point thereof may be determined.

A still further object is to provide a new and improved electronic recorder driver adapted to effect accurate control of the recorder.

Additional objects and advantages not speciiically set forth hereinabove will be apparent from the following description, taken in connection with the accompanying drawings, of which:

Fig. l is a view in perspective of the recorder driver;

Fig. 2 illustrates in diagrammatic form the complete electrical system of the recorder driver;

Fig. 3 is a view in perspective of the control device of the present invention according .t l the 'preferred embodiment thereof;

Fig. 4 illustrates in diagrammatic form the complete electrical system of the device of Fig. 3;

Fig. 5 illustrates in graphic form the variation in plate current of each one of the tubes in the recordel driver caused by a change in the grid vbias voltage of each one of the tubes;

Fig. 6 illustrates in graphic form the variation in plate current due to changes in the blocking potential applied to the device 'of Fig. 3; and

Fig. .'1 shows in diagrammatic form the fundamental grid blocking circuit of the pr-esent invention.

Referring now to Fig. '7 of the drawings the numeral iD designates a five element thermionic tube comprising a plate l l and heater type cathode l2. A screen grid 44 and suppressor grid l5 are tied to the plate and cathode respectively in the usual manner. Operating potential .is lsupplied to the plate li and cathode l2 from a battery iB. The control grid I3 of tube lo is disconnected or left floating with one plate Il of an air condenser being connected thereto. The second plate i8 of the condenser is arranged parallel to plate i7 thereof and supported by any suitable means which assures a high resistance path between grid i3 and cathode l2. The outer confdenser plate i8 is connected to a suitable source of A.C. supply voltage through a potentiometer I 9 which is also connected to cathode l2.

As the value of A.C. potential applied between the outer condenser plate and cathode is varied,

the charge in the condenser` comprising the plates .I1 and i8 and the bias of grid I3 will vary in accordance therewith in a manner heretofore described. As the grid bias varies, the amount of current flow in the plate circuit of tube In will vary in accordance therewith and cause a meter 2l, connected in the plate circuit, to deect to -give an indication of the magnitude of change inthe applied A.C. potential. It will be appreciated that the capacity of the condenser is very small in order that small changes in the applied A.C` potential may be detected and made manifest by the action of meter 2l. However, the capacity of the condenser may be changed by varying the distance between the plates i1 and I8.

.In order to prevent ambient electro-static fields from reacting on the plates tofvary the condenser capacity, tube l0 and the condenser are enclosed in a suitable cabinet 22.

Referring to Fig. 6, it will be seen that with applied A.-C. potentials between l0 and approximately Si) volts, the plate current flowing through tube Il) decreases very rapidly by reason of the increasing blocking potential appearing on the grid of the tube, the potentials on the plate and cathode thereof being held constant; As the applied A.C. potential is increased over 80 volts vless change is produced in the plate current, the curve becoming a nearly straight line. From an analysis of the curve of Fig. 6, it readily will be appreciated that the fundamental circuit of Fig,

present in cathode followers.

yfrom the A.C. source.

'7 may be utilized as a control device either oy shielding the circuit and causing it to be responsive to variations in the applied potential or by holding the applied potential constant and expos-v ing the condenser plates to the surrounding atmo'sphere to be responsive to changes therein such, for example, as changes in humidity which ai'ects 'the charge on the condenser. In the latter application, the condenser plates are separated further in order to decrease the capacity of the condenser and increase the sensitivity of the device.

An application of the grid blocking potential principle set forth hereinabove is illustrated in Figs. 3 and 4. The numeral 23, Fig. 4, designates a thermocouple which is adapted to control the operation of a galvanometer 24 variably in accordance with temperature changes detected by the thermo'couple. Blocking potential is applied to the control grid 25 pf tube 26 by a condenser `system ycomprising plates '21, 2S and 29. `Plate 2l is secured to the control grid 25 of tube 215 in any suitable manner and plate 28 is arranged parallel to plate 2 :and is supported in a manner adapted to provide a high .resistance path from grid 25 to the cathode 3l of tube 25. [nA-C. potential.` provided by secondary winding 3d of va transformer 46 having 'a primary winding 5E] connected to a suitable A.C. source, is appiied between plate 28 and cathode of tube 26 whereby the condenser formed by plates 2 andv 28 becomes charged to a value dependent upon the separation of the plates and the :magnitude of the vapplied voltage. Concurrently with the charge b'eing built up on the condenser, the vgrid of 'tube 26 becomes negatively charged to a value dependent upon the charge on the condenser. Inasmuch as potential is applied to the anode 32 of tube 26 from the A.'C. source, ya small amount of current will iiow therethrough dependent upon the potential on 'the grid.

The condenser plate 29 is carried by the indi eating arm 33 of the galvanometer '2li and is tied to ground through the arm, the normal nnoperated position of the arm being such as to position plate 2S away from plates 2l 'and 2S in order that the capacity therebetween will not be affected by plate 29 until the plate 29 is moved between plates '2l and 28, thereby providing critical amplitude controlover the apparatus to be operated in response to predetermined changes in the condition to be controlled. As the arm is deflected' in response to a temperature change detected by the thermocouple, plate 29 is adapted to pass between plates '21 and 28 thereby reducing the capacity therebetween and the negative charge on grid 25 to increase the flow of current through the tube.

A load resistor 34 is connected in the cathode circuit of tube 26A whereby the tube is adapted to be operated as a cathode follower and insure more stable operation of the control device by reason of the high grid to cathode resistance A 'tube 35 is employed in the control device and comprises an anode 36, cathode 37 and control grid 3B, potential being applied to the plate and cathode thereof A iixed bias for the control grid 38 of the ampliiier is provided by battery BA while an operating voltage is applied thereto from the cathode resistor 315 when the current flow through tube 25 is increased. Tube 35 is also cathode loadedV and is provided with a variable resistor 39 in the cathode circuit thereof, the potential drop produced across this resistor by an 'increase in plate current of tube 26 being applied to the control grid 4| of a gas type tube 42 by way of bias battery BAI to initiate conduction thereof. Such an arrangement yields a trigger action and allows critical amplitude operation in controlling circuits. Tube t2 further comprises an anode 43 and cathode 44, normal operating potential for these tube elements being supplied from the A.C. source. A resistor 45 is included in the cathode circuit of tube 42 to further insure stable operation of the control device.

Arranged in the anode circuit of tube 62 is an electrical relay 46 comprising stationary contact 41 and armature contact 48 and which, when operated, is adapted to complete a circuit from battery BAZ to a bell 48 thereby to give an audible indication of the temperature change detected by the thermocouple 23. A condenser I is shunted across relay 46 in order to prevent the relay from chattering, the condenser being shielded to prevent its electro-static iield from affecting the grid condenser. The heaters of tubes 26, and 42 may be supplied from secondary winding E0 of transformer 40 by any suitable circuit arrangement.

In operation of the control device, the A.C. potential applied to the condenser plate 2B pref erably is increased to a value which will reduce the normal current flow through the tube to near cut-off, and the tube is operated on the lower straight portion of the curve of Fig. 6 whereby small changes in the applied voltage due to imperfect regulation, tube leakage and capacitance variations are prevented from aifecting the operation oi the control device.

The change in plate current in tube 2B which is dependent upon a change in the capacity of the condenser formed by plates 21 and 28 may be increased by orientating the meter indicating arm in such a manner as to cause plate '2S carried thereby to pass closer to plate 21. Overall operating conditions may be greatly improved by 1ncreasing the length of the indicating arm in order that the arc through which the arm swings Will be increased whereby the meter movement may be placed at a greater distance from tube 26 and stray capacitance therebetween reduced to a minimum.

' Fig. 3 shows the general arrangement of the control device heretofore described. Tube 2G and its associated socket 52 are mounted on any suitable chassis. The grid condenser plates preferably are formed of tinned copper wire, plate 21 being secured, as by soldering, to the grid connection of the tube. Plate 28 extends through and is supported by a suitable insulating post-53, the position of plate 28 relative to plate 21 being maintained by a screw 54 mounted in the top of post 53 and the conductor leading to plate 2.8 preferably being shielded. Galvanometer 24 is arranged in a suitable cabinet 55 provided with a slot 5t through which the indicating arm or the meter extends whereby the meter elements are prevented from affecting the capacity of the grid condenser. Plate 29 is composed of any suitable liglit weight material such, for example, as aluminum tubing and is sleeved about the outer end of the indicating arm. The position of plate 2B relative to plates 21 and 28 is maintained by a base 20 upon which cabinet 55 rests. If it is desired to increase the effect of plate 2S upon the grid condenser, the outer end thereof may be lflattened as illustrated in Fig. 3.

' Figs. 1 and'2 illustrate an adaptation ci the grid blocking principle to a recorder driver system. Two similar tubes 51 and 58 are arranged in a balanced bridge circuit, each of the tubes forming one of the arms of the bridge circuit and comprising an anode 59, a screen grid 6l tied to the anode, a control grid 62 and cathode 63, The remaining two arms of the bridge circuit are formed respectively by variable resistors 64 and 65 connected in the cathode circuits of tubes 51 and 58 respectively. The detector arm of the bridge circuit is formed by a suitable polarity responsive pen recorder B6.

Blocking potential is applied to the tubes in a manner similar to that heretofore described. 'Each of the tubes is provided with a grid condenser comprising a common stationary plate 61 connected to a suitable A.C. source through a transformer 15 and a variable resistor 16, plate being suitably secured to the control grid of tube 51 to form the condenser therefor and plate (it being suitably secured to the control grid of tube 58 to forin the condenser therefor. A movable plate 1i is employed and is carried by a grounded indicating arm 12 of a galvanometer 13 arranged in a suitable cabinet 14, Fig. l, in order to prevent the moving elements of the meter from airecting the value of the blocking potential.

Operating potential is applied to the anodes of tubes 51 and 58 from a battery BAS, the cathodes of the tubes being tied to the negative side of battery BAS. A variable resistor 11 is arranged intermediate battery BAS and the anodes of the tubes in order that the potential applied thereto may be adjusted to cause equal values of current to flow through each tube when the movable plate 1I is in a null position intermediate plates 53 and 69. Suitable ammeters 18 and 1S are interposed respectively intermediate the resistor 11 and the anodes of tubes 51 and 58 for convenience in equalizing the current now in each of the tubes. For convenience in adjusting the pen of recorder 66 to the center of the record paper, an ammeter 8| is arranged in the detector arm oi the aforesaid bridge to indicate a zero current therein.

Referring now to Fig. 5 of the accompanying drawings, there is shown thereon the operating characteristics of tubes 51 and 58, curve A representing the characteristics of tube 51 and curve B the characteristics of tube 58. With a predetermined A.C. potential applied to the stationary plate Sl' and a predetermined separation between each of plates 68 and 69 and the common plate 51, the swing of the grid potential applied to each one of the tubes in accordance with a full deiiection of the movable plate 1l in either direction from the null position thereof will be limited in such a manner to cause the plate current change in either of the tubes to be linear, as illustrated in Fig. 5.

For the purpose of description, let it be as-v sumed that the system of Fig. 2 is to be employed to record the output of an induction pick-up coil 82 in response to a changing ambient magnetic field in which case the galvanometer 13 is connected to the output terminals or the coil. Let it further be assumed that as the intensity of the ambient magnetic field increases over the normal value thereof, the indicating arm or the galvanometer and condenser plate 1| carried thereby is caused to move in the direction of condenser plate 8. When this occurs, the capacitance between plates 61 and B8 and the negative charge on the control grid 62 of tube 51 is reduced, thus increasing the value of current flow in tube 51. As

s, masas the: current.. flow throughztubel 521 increases, they potential drop produced across.' the cathoder resist'or P54 fthereoi is increased;` whichipotentialfinfcrease is' appliedxto the feld ofthe-recorder driv.-

ing motory tok causezthel inking pen'v thereof to recordthechangein .the Aambient Vmagnetic eld in. a Well known, manner.:4

TheV potential applied. tothe-field. ofv the recordermotor increases asi them'eter indicating arm and condenser plateh carried thereby continue to move inthe direction of plate 68 in response to a continuous" increase.v in intensity. of the ambient magneticy field;y maximum motoreld potential being reacliedxvvhenv plate: ll is posi.- tionedfdirectly between plates 6.1 fandzli. As the intensity of the'--ambientina-ghehe.y field decreases from this value, the-meter indicatingarm and plate carri-edy therebyfarecaused; to moveftoWard the null positionthereoi, therebyfcausing an increase in capacity betweenplateszland 6B and in the Vnegative charge appearingzonthe control grid oftube 5l. Whenthis occurs, the current iioW through thetube decreased as isthe potential drop across resistor til whereby the pen of recorder t6 is'causedltoxmove towardthe null position thereof; this action continuing as the magnetic eld'approaches its normal strength. It readily Will be understood, from the foregoing description, that inthe event'that the intensity oi the magnetic eld decreases fromV thenormal value thereof, the meter armand condenser plate carriedthereby are caused to move toward'condenser plate; S9* to increase; the current' now through tube 53 and-the kpotential drop across the cathode resistor 65; thereofthereby to'causezthe recorder pen and motor to `operate ina direction opposite to that caused by the increase in intensity of the. magnetic eld.

Since a bridge circuit-is. employed in therecorder cniver;v system,y any changes inthe sur;- rounding atmosphere such, for example; as changes in humidity, will aiect bothtubesfto the same extent and` will. be canceled: out. In the event that greater linearity.A betweenthe input to meter 'i3 `and the current' change-in either tube is;- desired, adjustments on. the:y A1-C'. potential applied to stationary plateis'zand onrthe separation betvveenzthe movable plate andplateslisand Gil-may be made. it willibet understood, ofcourse,-

that trigger. action is-to be avoidedgin this appli'- cationv of the blocking: potential principlerand; therefore,` the separation between plateszzand 6,3 andthe `stationary plate lzisggreaterthanthat inthe relay application.

From the foregoing, it shouldnowbe. apparent that' several. embodiments of. the. invention have been provided' which' are well' adapted to; fulfil the `aforesaid objects of the, invention..

While the; invention has been' describedy with particularity in reference to examples; thereof which give satisfactory results, it readily will. be apparent to those skilled in the art; afterl under;- standing the invention, thatzfurther embodiments, modifications and` changes mayvbe. madetherein without departing from; the spiriti and. scope thereof as deiinedxby the claims appended hereto.

The present invention may be manufactured and used by or for the Government of the-United States of America for governmental purposes Without payment of` any'y royalties thereon; or therefor.

What is claimed-as newand; desired to be; se.- cured by Letters Patent. of the United States-is:

l; A. recorder driver system of the-character disclosedcomprisingin, combination; apair of aircondensers havingia;commonixedplate. and" acommon movable plate; a source-.of A.-G. potential. constructed and arranged to chargev said condensers through said; common iixedv plate, means responsive; to:y changes in a physical condition for.: moving ,said movable plate vvith` respect towsaid common fixedplatevariably in accordanceV vvithv said changes thereby to vary. the capacity of either oneA of said condensers` selec,- tively in accordance withv deviations-of the physical condition from; al predetermined value.; thereof, and a vacuum tube bridge circuit having-,two electron discharge p aths individual to saidcondensers and controlled by. the capacity thereof,

saidbridge circuitalso includinga polarity sensitive recorder connected between said discharge paths as the detector army of. the bridge circuit. andpconstructed-andarranged tobe operated in either direction` selectively in accordance With the variations inthe capacitance` oi. the condenser beingI aiected-by said condition responsive means.

2. A recordery driver system of the, character disclosed comprising, in combination, means responsive to changes in. aphysical condition, a galvanometer controlled by said. condition responsive meansand having an indicating arm delectable ineither directioniromanull position upon changes in the condition, `a pair of electron discharge devicesarrangedto,formtwo arms oi' a balancedbridgecircuit; each one of said discharge devices having a-,floating control element anda maindischargepath the current in which is controlled by thervoltage on the control element, a sourceof constant A-C potential, means including a pair of-condensers. of` changeablecaf pacitance soconstructed and arranged' as to be charged from said A.C.. source and respectively connectedv to the control elements of said` discharge devices to supply'voltagesthereto which varyin value in-accordance with changes in the capacity of the condensers` respectively, a pair of. impedance elements respectively. arrangedV in the discharge paths of.4 said discharge devices andk forming; thei two remaining arms of said bridge circuit, polarity responsive recording means arrangedv asl the. detectorv arm of said bridge circuit and constructedand arranged to be operated selectively in either direction in accordance with the-.polarity of thevoltage difference, appearing; across said. impedance elements, and means constructed andm'ranged for coaction with said condensers and ,carried by said indicating arm for changing the capacitance of either oney of said condensers` selectively in. accordance with an increase or decrease in said physical condition.

3; A triggentype electroniccontrol system comprising, in combination, an electronic discharge devicel havingoating control grid and cathode elements which are broughtlout of` the discharge device in Widely spaced relation to provide 10W surface leakage therebetween, said discharge device also having a. main electron discharge. path the-current in whichiscontrolledby. the voltage on` the control gridelement, anair condenser of variable capacitance-vv comprising., apair of Wirelike plates secured .in closely. spacedparallel rela,- tion and a third. Wire-like plate movable into a position between said pair of plates in parallel relation therewith thereby to` greatly` change, the capacitance of thecondenser, one of said pair of plates being securedto said grid. control. element substantially. at the. point. of, emergence thereof fromsaiddischarge, device, agalvanometer having an elongated indicating arm electrically connected to the cathode element, said movable plate being carried by said indicating arm and movable thereby into said position between said pair of plates from a position widely spaced therefrom when the galvanometer is operated by a current of predetermined strength, a source of constant A.C. potential interconnected between said cathode element and the other one of said pair of plates for applying a voltage to the grid control element which biases the current in said discharge path closely to `cut-wort, and electroresponsive means connected in said discharge path and operable in response to the current 110W- ing therein when said movable plate moves into said position between the pair of plates.

4. A stepless type electronic control system comprising, in combination, a pair of vacuum tubes each having floating control grid and cathode elements brought out of the tubes individual thereto in widely spaced relation to provide low surface leakage between the elements, each of said tubes also having a main electron discharge path the current in which is controlled by the potential on the control grid element individual thereto, a pair of wire-like substantially parallel condenser plates secured respectively to the control grid elements of said tubes substantially at the points of emergence thereof from the tubes,

a third condenser plate secured midway between said pair of plates, a fourth condenser plate movable between said pair of plates substantially in parallel relation therewith thereby to vary the capacitance between said third plate and each of said pair oi plates as the fourth plate moves y therebetween, a pair of resistances connected in series between the cathode elements of said tubes and respectively within the discharge paths thereof for causing a difference in potential between the cathode elements which varies linearly with the changes in position of said fourth plate, a galvanometer having an elongated indicating arm electrically7 connected to the junction of said resistances, said fourth plate being carried by said indicating arm and movable valiably thereby between said pair of plates in accordance with the changes in the magnitude and polarity of the galvanometer driving current, a source of constant A.C. potential interconnected between said third plate and said indicating arm for ap- 1 plying a predetermined bias voltage to said control grid elements in accordance with the amplitude of the A.-C. potential, and electroresponsive means interconnected between said cathode elements and operable linearly with respect to said n 10 iioating control grid and cathode elements which are brought out of the device in widely spaced relation to provide low surface leakage therebetween, said discharge devices also having main discharge paths the current in which is controlled by the voltage on the control grid elements thereof, an air condenser formed of a pair of wire-like plates so constructed and arranged as to render the condenser variable in capacitance in response to changes in ambient conditions, said wire-like plates being respectively secured to said control grid elements substantially at the point of emergence thereof from the discharge devices, a xed plate interposed intermediate said plates and disposed laterally of the plane thereof, a source of A.-C. potential interconnected between the cathode elements and the fixed plate for applying a bias voltage to said control grid elements in accordance with the amplitude of the .11,-C, potential, a movable wire-like element constructed and arranged for movement between said wire-like plates in the plane thereof variably in accordance with changes in a preselected condition, means including said discharge devices which are similarly disposed for preventing response of the condenser to a humidity condition whereby the voltage on the control grid elements is varied in accordance with changes in the preselected condition and is not obscured by the humidity effect thereon, and electroresponsive means connected in said discharge path and operable variably in accordance with changes in the current therein thereby to make manifest said changes in the preselected condition.

EVERET'I' L. DEETER.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,112,549 Pierce Oct, 6, 1914 1,893,019 Culver Jan. 3, 1933 1,907,093 Shivers May 2, 1933 1,980,816 McDill Nov. 13, 1934 2,089,677 Kramolin et al Aug. 10, 1937 2,117,894 Lenehan May 17, 1938 2,165,510 Rosene July 11, 1939 2,284,476 MacKay May 26 1942 2,309,560 Welty Jan. 26, 1943 2,404,307 Whitaker July 16, 1946 2,416,780 Tellier Mar. 4, 1947 2,449,068 Gunn Sept. 14, 1948 FOREIGN PATENTS Number Country Date 505,906 Germany Aug. 27, 1930 OTHER REFERENCES Morecroft, Principles of Communications, John Wiley 8: Sons Inc. N. Y., 1921, pgs. 410-415. 

